The film-forming or film-depositing technology that utilizes the sputtering phenomenon caused by glow discharge under a reduced pressure in the presence of dilute gas is now used in a broad range of applications including technological devices and equipment, electronic devices and equipment, and ornaments or decorations. As regards the sputtering targets used in that technology, many proposals have been made, and various types of targets suited for the individual applications are in use.
Sputtering targets usually are disk-shaped, and sputtering is carried out with each sputtering target 1 bonded to a water-cooled backing plate 2 of copper, as shown in FIG. 4.
As sputtering is continued, the target is consumed with the lapse of time, as indicated in FIG. 5. When the point where erosion has progressed most severely reaches a location near the bottom of the target, the life of the target is deemed to have expired and the target is replaced by a new one.
If the time for replacing the sputtering target is too early, such a substantial part of the target is left unused that the target efficiency is poor. Conversely, if the time for replacement lags, the target efficiency is improved but erosive attack of the backing plate (over-sputtering) can take place. For the practice of sputtering, therefore, precise judgment of the sputtering target life has been a very important requirement to be considered.
Whether or not a sputtering target in service is nearing the end of its life can hardly be directly determined visually or otherwise. It has been customary to use, as a measure of sputtering target life, the integrated electric power value applied to the target, which value is closely related to the consumption of the target.
Specifically, with different types of sputtering targets the relations between their integrated electric power values and degrees of erosion are determined beforehand on the basis of actual past operation results. From these relations, the critical values of integrated electric power that cause over-sputtering are calculated. At the point where the integrated electric power value during sputtering with a given target has reached a preset value near its critical value, the target life is judged to have expired and the target is replaced by a new target.
However, this practice suffers from the problem that determining the optimum preset value is difficult.
As stated already, the integrated electric power value indicative of the time for replacement of a sputtering target has commonly been set in view of the relation between the integrated electric power value and the degree of erosion based upon past operation results. In reality, even target materials of the same kind often vary in density or in crystalline orientation and hence are eroded to varying degrees. Thus, strictly speaking, the relation between the integrated electric power value and the degree of erosion differs more or less between individual sputtering targets.
There are few steps that can be taken to overcome the low efficiency of some sputtering targets. Not infrequently, an over-sputtering accident has occurred. The generation of defective target products and damage to manufacturing equipment due to over-sputtering, of course, involves a very large loss.